Flutter video_player with URL from Firestore Document

I'm trying to play a video from a URL of a Firestore Document. To play a video in Flutter, I have to instantiate its Url in the init() method. I set a default URL to a butterfly video, and the value was supposed to be replaced by the URL obtained from Firestore. (So that it is easy for me to see if the code works). However, the code does not work properly. I got an error that says "NoSuchMethodError: The getter 'value' was called on null".

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:video_player/video_player.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // Create the initialization Future outside of build
  final Future<FirebaseApp> _initialization = Firebase.initializeApp();

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
        future: _initialization,
        builder: (context, snapshot) {
          // Check for error
          if (snapshot.hasError) {
            print(snapshot.error);
            return Center(
              child: Container(
                child: Text(
                  "Something went wrong",
                  textDirection: TextDirection.ltr,
                ),
              ),
            );
          }

          //Once complete, show your application
          if (snapshot.connectionState == ConnectionState.done) {
            return MaterialApp(
              title: 'Flutter Demo',
              home: VideoPlayerScreen(),
            );
          }

          return CircularProgressIndicator();
        });
  }
}

class VideoPlayerScreen extends StatefulWidget {
 
  @override
  _VideoPlayerScreenState createState() => _VideoPlayerScreenState();
}

class _VideoPlayerScreenState extends State<VideoPlayerScreen> {
  VideoPlayerController _controller;
  Future<void> _initializeVideoPlayerFuture;
  FirebaseFirestore firestore = FirebaseFirestore.instance;
  String videoUrl =
      'https://flutter.github.io/assets-for-api-docs/assets/videos/butterfly.mp4';

  @override
  void initState() {
    firestore.collection("videos").get().then((QuerySnapshot querySnapshot) => {
          querySnapshot.docs.forEach((doc) {
            // _controller.dispose();
            videoUrl = doc["videoUrl"];
            _controller = VideoPlayerController.network(videoUrl);
            _initializeVideoPlayerFuture = _controller.initialize();
            print(videoUrl);
          })
        });
    // _controller = VideoPlayerController.network(videoUrl);
    // _initializeVideoPlayerFuture = _controller.initialize();
    super.initState();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Flutter Video Player"),
      ),
      body: FutureBuilder(
        future: _initializeVideoPlayerFuture,
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            return Column(
              children: [
                AspectRatio(
                  aspectRatio: _controller.value.aspectRatio,
                  child: VideoPlayer(_controller),
                ),
              ],
            );
          } else {
            return Center(child: CircularProgressIndicator());
          }
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          setState(() {
            if (_controller.value.isPlaying) {
              _controller.pause();
            } else {
              _controller.play();
            }
          });
        },
        child: Icon(
          _controller.value.isPlaying ? Icons.pause : Icons.play_arrow,
        ),
      ),
    );
  }
}